In a fuel supply system in which a high-pressure pump pressurizes a fuel and supplies the fuel to a fuel injector, a fuel filter is provided upstream of the high-pressure pump. The fuel filter removes foreign matters contained in the fuel. When the fuel filter is clogged with the foreign matters, an efficiency of filtration is deteriorated. A pressure loss is increased in the fuel filter, and a fuel flow rate may be decreased.
JP-2011-122518A shows a fuel supply system which is able to detect a malfunction due to a clogging of a fuel filter. Specifically, an electric current value flowing through an electric pump or a rotation speed of the electric pump at an idling state of an engine with no malfunction is compared to that with a malfunction. Based on the compared result, the system determines whether a malfunction exists or not.
In a fuel supply system in which an electric pump supplies the fuel to the high-pressure pump, a malfunction in a fuel filter can be detected based on the electric current value flowing through the electric pump or the rotation speed of the electric pump. However, even in a case that a fuel supply system does not have an electric pump, it is desired to detect a malfunction of the fuel filter.